The so-called “Terahertz Gap”, between 0.3 to 10 THz, is a frequency range where a plethora of potential important applications exist, while there is a paucity of sufficiently powerful sources to exploit these applications. Opportunities in the use of terahertz radiation exist in basic science, in medicine and in the government sectors of the economy. Transportability may be an important requirement, given some of the military uses of terahertz power.
The obstacles that have stood in the way of developing practical terahertz sources have been the fundamentally low efficiency and output power of the solid-state devices that have been employed to date to produce terahertz radiation in portable or practically transportable systems. Although quantum cascade devices have been extended from the near-infrared region to somewhat less than 2 THz, issues exist, including how far below 2 THz they may operate and whether cryogenic temperatures are required for high performance. Existing portable solid state-based devices are limited in power, which is typically on the order of milliwatts. On the other hand, the size and cost and power requirements of FEL (free electron laser) or laser-based terahertz sources, which may provide hundreds or even thousands of watts of power, are, in most cases, prohibitive for the intended use.
An alternative to solid-state devices and photonic devices are conventional microwave tubes. These are available as oscillators only, but the average power they produce measures well below 1 watt. Problems with these devices are the removal of waste heat from the small areas where it has to be generated, and the formation and confinement of a beam with sufficient current, given that the beam cross-section area is proportional to the square of the wavelength. In addition, the fabrication of the RF (radio frequency) interaction structures required presents increasingly serious difficulties with decreasing wavelength.